Natural gas is increasingly becoming an attractive alternative for fueling internal combustion engines. In one specific example, a compression ignition engine is fueled predominately with natural gas originating from a gaseous fuel common rail, and liquid diesel fuel from a liquid fuel common rail that are directly injected into each engine cylinder. Both fuels are injected from the same fuel injector, and the relatively large charge of gaseous fuel is ignited by compression igniting a small pilot injection quantity of liquid diesel fuel. When both fuels are injected from a single fuel injector, there is a possibility of gaseous fuel migrating into the liquid side, and vice versa, which can lead to undermining proper operation of the fuel system. Co-owned U.S. Patent Application Publication No. 2012/0285417 shows an example of such a dual fuel system. During typical operation, the liquid fuel pressure is maintained greater than the gaseous fuel pressure to inhibit migration of gaseous fuel into the liquid fuel side of the fuel system. Pressure in the liquid fuel common rail can be changed quickly due to the relative incompressibility of the liquid fuel. However, changing pressure in the gaseous fuel common rail is far different due to the highly compressible nature of the gaseous fuel. Reducing pressure in the gaseous fuel common rail during a load loss transient without venting substantial amounts of gas to atmosphere can be difficult, while maintaining a pressure differential that inhibits migration of gaseous fuel into the liquid fuel side of the fuel system.
The present disclosure is directed toward one or more of the problems set forth above.